This invention relates to sensors for monitoring the position of a door or the like; and, more particularly, to a tandem sensor for use in such monitoring and which cannot be readily defeated.
In security systems for monitoring a premises, a sensor is often used with a door or window to provide an indication when the door, for example, is open when it shouldn't be. One type of sensor used for this purpose utilizes a reed switch. As shown in FIG. 1A, a door D is monitored by a sensor S comprised of a first element E1 attached to the door and moving with the door as it opens and closes, and a second element E2 which is permanently affixed to a door frame or jamb F. In FIG. 1B, element E1 is shown to include a magnet M1 which is installed in a housing mounted on the door. Element E2 is shown to include both a magnet M2 and a reed switch RS both of which are installed in a housing mounted to the frame. When the sensor is installed, the elements E1 and E2 are positioned relative to each other so when door D is closed, the magnetic fields produced by magnets M1 and M2 cancel each other out at the location of reed switch RS and switch RS is in a null field. In this position, switch RS is deactivated as shown in FIG. 1B. When door D is opened, element E1 moves relative to element E2 and the magnitude of the magnetic fields to which switch RS is subjected change. Now, the switch is no longer in a null field, but rather, the net magnetic field to which it is subjected will cause the switch to activate. If an alarm system in which the sensor is incorporated is activated, this action will cause the system to go into alarm.
While this sensor configuration is commonly in use, it is possible, although extremely difficult, to defeat the sensor. Referring again to FIG. 1B, assuming door D opens in the direction of the arrow, if a third magnet M3 is introduced and if this third magnet is appropriately positioned as the door opens, the net magnetic field to which switch RS is subjected will remain essentially the same as that when the door is closed; i.e., a null field. In a null field, the reed switch will not activate, and the system will not go into alarm, even though door D is opened. Accordingly, the security of the premises can be breached and no one will know it while it is happening.